This invention relates to closures for cans and more particularly to closures for sanitary or food cans which are subjected to high temperatures and pressures during the retorting process. These can ends must be designed to withstand the pressures developed within the can during processing and pasteurization to avoid permanent distortion or buckling. The ability of the end to withstand these forces depends not only on the proper selection of the plate composition and thickness but upon the design of the end as well. A downwardly directed annular bead is introduced concentric with and inwardly adjacent to the countersink groove. This bead reinforces the end adding strength and rigidity to enable the end to resist or even overcome any buckling tendency. For the packaging of dog food which is subjected to severe processing conditions can ends with a pair of concentric down beads have occasionally been employed. The ends are further contoured with a series of transitional steps or panels inward of the bead. These steps contribute to the pull back or spring back strength of the end which enable an end which has bulged to pull back into shape when the pressure gradient across the end has been reduced as for example when the retorted can is cooled.
End distortion can occur during service as well as during processing. For example cans packaged with dog food in a west coast cannery at sea level and shipped over the Rocky Mountains to Denver were found to be permanently distended. The distended end makes the can look as if the contents were spoiled, when in fact the real problem was to increase the end strength to better resist distortion under the low atmospheric pressures encountered in shipment.
In the continuing efforts of container manufacturers to improve material efficiency or utilization there has been a trend toward the use of stronger but lighter weight plate such as DR8 or DR9. This plate is double cold reduced where the steel plate is subjected to two cold working stages rather than the single stage which is conventionally employed. The resulting product is harder and has a higher tensile strength than conventional plate but it is also less ductile. As a result when a sanitary closure is formed from light basis weight plate, such as 75 lbs/BB or 85 lbs/BB, (BB=31360 square inches) the beading and paneling convolutions which strengthen the end serve to generate the collateral problem of end warpage.
Such warpage occurs in the unseamed end and is independent of the distortion which may occur during product retorting or in subsequent service. The warpage appears to result from the unbalanced stresses which are introduced into the end during manufacture. Thus by drawing a high reenforcing bead in an end panel fabricated from high strength light weight plate, the rididity and strength may be achieved, but at the sacrifice of planarity.
A warped end if placed flange down on a flat surface will contact the surface along only a portion of its periphery, whereas an unwarped end being planar will contact the flat surface around the entire periphery. While modest warping can be tolerated, excessive warping result in end seaming and end stacking problems. The present invention addresses the problem of strengthening the end without inducing excessive warpage and further recognizes the need for such a solution to be compatible with basic requirements such as openability.
Accordingly it is an object of this invention to provide an end fabricated of light weight high strength steel plate which is suitable for use as a closure for a sanitary can. It is further an object of this invention to provide an end which has been strengthened to resist permanent distortion in service, but where the stresses resulting from the strengthening are balanced to avoid inducing warpage of the unseamed end.
Finally it is an object of this invention to provide an end which has been strengthened to resist distortion in service, but where the end conforms to conventional practice so that the resulting can may be readily opened with a conventional can opener.